Heat treatment of expansible materials to form lightweight aggregate

ABSTRACT

A rotary kiln (10) is employed which is inclined downwardly from its charge end (12) to its discharge end (14) and has a maximum length to breadth ratio of 5:1. The interior space of the kiln (10) is heated by means of at least one burner (B1, B2) directed into the kiln from one end thereof to a substantially constant elevated process temperature. Material is fed into the kiln (10) so that it occupies a maximum of 10% of the cross-sectional area of the internal space of the kiln (10) at the charge end (12). Upon entering the kiln the material is subjected almost immediately to the required process temperature. As it moves along the bottom of the kiln (10) from the charge to the discharge end (14), heat transfer to the material is primarily by radiation from the combustion space and from the lining of the kiln (10).

There is considerable demand for lightweight aggregate for use by theconstruction industry in lightweight aggregate concretes. Such concretesare used in the form of building blocks, precast components such aspanels, beams and roofing slabs, and in many other applications wherelightness coupled with sufficient strength and low thermal conductivityare important. There are also applications where the aggregates can beused for insulation alone for example as space infill below suspendedground floors, fillings for hollow cavities and so on. Them are yetother uses, in horticulture for instance, where lightweight aggregatesare used in composts and as a growing media in hydroponic cultivation.

It is well known that lightweight aggregates can be produced bysubjecting certain suitable natural materials, such as some clays,shales and slates, to heat treatment at elevated temperatures to bringabout their expansion. Where such expansion is possible, the material tobe expanded usually has to be in particulate form as slate or shalechippings, or as pellets made from finely ground clay, slate or shale.

In this invention much consideration has been given to thephysical/chemical mechanism whereby materials are expanded and its aimis to provide as far as practicable conditions whereby maximum expansioncan be achieved. For expansion to take place the material has to beheated to the point of incipient fusion. At these high temperatures thematerial becomes plastic and impervious and the gases that are thenproduced within the material are prevented from escaping. This resultsin the formation of a myriad of small gas pockets which cause theplastic material to expand and on cooling the porous structure remainswithin the solidified material. Some gases are evolved within thematerial at temperatures substantially lower than incipient fusion and aprolonged period of heating to incipient fusion allows these gases toescape uninhibited. It is a teaching of this invention that the materialto be expanded should be heated rapidly to incipient fusion, and that itshould only be held at this expansion temperature for a time sufficientfor full expansion to take place as unnecessary exposure at an elevatedtemperature can lead to deterioration and a weakened structure.

A proportion of expansion gases, mainly steam from the water ofcrystallisation of the mineral constituents are evolved at comparativelylow temperatures, below 500° C. Thus preheating the material up to say500° C. prior to the rapid heating process will detract to a degree fromthe expansion obtained, but this may be desireable in the interests offuel economy where such preheating can be effected by using waste heatwhich would otherwise pass out of the system.

Existing commercial processes for producing expanded lightweightaggregate from natural materials have centred principally on two methodsof heat treatment, namely using a sinter strand or travelling grate, orusing a rotary kiln. Both methods involve heating large quantities ofmaterial to elevated temperatures comparatively slowly.

To put this invention into perspective it is necessary to considerexisting rotary kiln practice. The rotary kiln consists of a tubularvessel, lined with refractory and insulating material, which can rotateabout its longitudinal axis. The kiln is inclined to the horizontal,e.g. by up to 10 degrees, so that when it is rotating any particulate orother material fed into it at its upper end will gradually progress downthe kiln until it is discharged at its lower end. Heat is supplied tothe discharge end by burners firing directly up the kiln or perhaps byglancing the flames on to the walls or the roof of the kiln. Transittime for material to pass through the kiln is governed by the speed ofrotation and the angle of inclination. Material travels incountercurrent against the direction of flow of combustion gases. Thereis a substantial temperature gradient along the length of the kiln suchthat at the discharge end the temperature attained is the fusiontemperature of the material (say 1200° C.) whilst at the feed end thetemperature may be in the range of 300° C.-600° C. Heat transfer alongthe kiln length is principally effected from gaseous combustion productsto the material and baffle means may be provided within the kiln toassist heat transfer by tumbling the material in the gas stream. Near tothe discharge end, however, heat transfer to the material is alsoeffected by direct flame impingement and by radiation from the kilnwalls. Thermal efficiency in terms of heat supplied to heat transferredsuccessfully to the material can only be further improved by lengtheningthe kiln and this is limited by capital cost and practicalconsiderations. Generally speaking the configuration economicallyconsidered sensible to construct involves a length to diameter ratio ofabout 15 to 1. In such commercial kilns the residence time of materialwithin the kiln is usually anything between 30 and 90 minutes with manytons of material under treatment within the kiln.

Having regard to the above, the conventional rotary kiln suffers fromthe following inherent disadvantages:

i) Poor thermal efficiency due to large surface radiation and otherlosses associated with the long length of the kiln, and combustionproducts leaving the kiln at too high a temperature at the feed end.With regard to the second, it is impractical and too expensive in termsof capital cost to lengthen the kiln to improve matters.

ii) The heating of the material to incipient fusion is too slowresulting in loss of expansion potential.

iii) Capital cost is very high due to the sheer size of plant even withthe normally accepted sizes (length to diameter ratio in the region of15:1).

The object of this invention is to produce fully expanded aggregate withminimum fuel consumption in apparatus of lower capital cost thanstandard rotary kilns. That is to say to eliminate the disadvantagesdescribed in (i), (ii) and (iii) above.

According to the invention a method of heat treatment of expansiblematerial to form lightweight aggregate is proposed using a rotary kilnwhich is inclined downwardly from its charge end to its discharge endand has a maximum length to breadth ratio of 5:1, said method comprisingheating the interior space of the kiln by means of at least one burnerdirected into the kiln from one end thereof to a substantially constantelevated process temperature and feeding the material into the kiln sothat it occupies a maximum of 10% of the cross-sectional area of theinterior space of the kiln at the charge end so that upon entering thekiln the material is subjected almost immediately to said processtemperature, heat transfer to the material as it moves along the bottomof the kiln from the charge to the discharge end thereof being primarilyby radiation from the combustion space and from the lining of the kilnand by direct contact with the lining of the kiln.

Advantageously, heating may be achieved by at least one pair ofregenerative burners directed into the respective ends of the furnace.

Thus, according to the invention a process is proposed whereby a rotarykiln is adapted and used in a manner characterised as follows:

(a) The length to breadth ratio is reduced to a maximum of 5:1 so thatheating means in the form of burners (with attendant flues and/oreductors as appropriate) at the discharge end, or the feed end, or atboth ends enable the temperature along the whole kiln length to bemaintained at an elevated process temperature. Such burners can beadjusted independently to bring about an isothermal chamber.

(b) At feed, material, whether preheated or not, is immediatelysubjected to the said elevated temperature.

(c) The feed rate of material is restricted so that at the feed station,in section, the feed occupies less than 10% of the total sectional area.

Transit time of material through the kiln is determined by a combinationof the angle of inclination of the kiln to the horizontal and the speedof rotation. Preferably it is less than 20 minutes, usually less than 10minutes.

For any chosen transit time, there is a maximum loading of materialwithin the kiln that can be successfully treated. Loading can be gaugedby reference to the kiln in section just after feed and the percentagesectional area occupied there by the unexpanded material (occupancy).The shorter the transit time the less is the occupancy that can betolerated. Once the transit time is set, occupancy is then controlled byadjusting the feed rate. Generally the transit time is chosen to be ofshort duration say between 2 and 8 minutes and the feed rate is thenadjusted to suit. This means that the occupancy at the feed station maybe reduced to well below 10% to, say, between 2 and 5% of the sectionalarea and sometimes even less than this, particularly when cold feed isused. Constant high temperature treatment of small quantities in rapidtransit through the kiln is the essence of the process.

The material under treatment is heated by radiation from a radiativecombustion space which is large in volume compared to the charge and bythe hot lining passing beneath. Heat transfer direct from combustiongases as takes place in a conventional rotary kiln is not intended andany such transfer is incidental.

Conditions within the kiln will involve the products of combustionleaving the kiln at high temperature, something approaching that of theprocess temperature. Heat is preferably recovered therefrom external tothe kiln and is used to preheat the combustion air.

If it is desired to improve fuel economy (albeit at the expense of someexpansion), the sensible heat contained in the hot product leaving thekiln may be used to preheat the feed material.

The kiln need not be strictly cylindrical, and could have a polygonalcross-section. Its length to breadth (or diameter) ratio is preferablybetween 2 and 4 to 1. For example, a typical ratio might be 2.5 in whichcase an internal furnace diameter of 1.5 m would be accompanied by alength of 3.75 m. The kiln is fired from either end or from both endsdepending primarily on the length to breadth ratio chosen. In practice,these burners with attendant flues or eductors will be positionedtowards the top of the furnace, i.e. approximately diametricallyopposite the position adopted by the material charge passing throughwhen the furnace is in motion so that the charge will be remote from thepassage of combustion gases.

As the kiln rotates the material tumbles and progresses towardsdischarge and is systematically exposed to an intense radiant flux fromthe combustion reaction itself and from the kiln lining. It is alsoheated by direct contact with the hot kiln lining.

The short furnace length as proposed by the invention is aimed atkeeping the furnace temperature constant along its length, as far aspracticable. There may, however, be temperature reductions (acceptableones) at the ends of the furnace due to heat losses at the dischargeaperture and a general cooling upon admission of fresh material at thefeed station. This cooling is minimised when at the feed stationoccupancy is kept small, e.g. below 5%, of the cross-sectional area ofthe kiln interior, typically 3%, but sometimes even less than this whenmaterial is fed in at ambient temperature.

Maintenance of a high temperature throughout the furnace, with thematerial being subjected to this immediately, or almost immediately itis fed in maximises expansion of the material particles. Gaseousproducts are trapped instead of being dissipated, as occurs duringslower heating in the previously known apparatus/method. The outersurface reaches fusion in minimum time, before expansion gases from theinterior can evolve and escape.

Also, because the furnace in accordance with the invention is short inlength and compact, there is a substantial reduction in capital costcompared to a conventional rotary kiln.

Inherent in the expansion process is the need for the material to softenand start to fuse. In this state it becomes sticky and the individualparticles start to stick together and form agglomerations and, moreseriously, the material whether agglomerated or not sticks to therefractory lining of the kiln. This sticktion to the refractory liningis known as "ringing" and has the effect of preventing an orderlytransit of material through the kiln and can even develop into acomplete blockage.

To obviate such undesirable effects two courses of action are possible.One course is to slightly lower the temperature of operation (say by25°-50° C. approximately) to prevent sticktion, but then the productleaving the kiln is not fully expanded. The other course is to add acohesion inhibitor, in the form of a powdered or finely sized materialwhich has a substantially higher melting point than the material undertreatment. It is important that this additive does not promote liquidformation by the formation of a eutectic with the material undertreatment. The use of fine fairly pure silica sand of granular size200-500 microns is ideal. The quantity of sand that needs to be addedwill have to be determined by experiment, and will depend on the sort ofproduct which is desired. If it is desired merely to permit material,whether agglomerated or not, to pass through the kiln, then onlyrelatively small quantities of sand will need to be added (say 10% ofthe weight of the material being treated) and the material will need tobe crushed and graded after treatment to produce aggregate of desiredsizes from expanded material which has also perhaps agglomerated. On theother hand, if it is desired to produce expanded aggregate as individualparticles each expanded in their own right then a larger proportion ofsand will have to be added with the feed.

When using the method of the invention sand or a similar finely dividedgranular refractory material will probably need to be admixed with thefeed, as a cohesion inhibitor, to minimize problems just outlined.

Such sand should preferably be separated from the product while hot andrecirculated back to the furnace as hot as possible to enhance fuelefficiency.

As previously mentioned, one way of putting the invention into effect isto employ a complementary pair of regenerative burners as heating meanswith one burner of the pair being directed into the feed end and theother into the discharge end.

The use of a regenerative burner pair (which is possible because thefurnace itself is so compact) allows combustion efficiency to beincreased to something approaching 80% with fuel savings of about 50%compared to conventional burners. In use, at any one time one burner ofthe pair is operatively supplied with a chosen combustible fuel/airmixture and applies its heat to the furnace space. The products ofcombustion, after passing across the said space, are taken up by thecomplimentary burner and passed to equipment which serves to retain theaccumulated residual heat contained in such combustion products. Uponthe accumulated heat reaching a predetermined level, the roles of thetwo burners of the pair are reversed, with the previously accumulatedheat each time serving to preheat the combustion air being supplied tothe operative burner. With a furnace temperature of 1200° C. thecombustion air can be preheated to 1000° C. using this system.

The combustion efficiency would be less than 40% using conventionalburners, i.e. with cold air and no preheat at all.

Two arrangements, and results that can be achieved, will now bedescribed with reference to the accompanying drawings in which:

FIG. 1 is a diagrammatic sketch of a kiln or furnace and associatedequipment employed in a process in accordance with the invention; and

FIG. 2 is a section along line A--A in FIG. 1; and

FIG. 3 is a diagrammatic sketch of a modified arrangement of equipmentwhich may be associated with a kiln similar to that shown in FIGS. 1 and2 and employed in a modified process in accordance with the invention.

The terms kiln and furnace as used herein are intended to have identicalmeaning.

Referring to FIGS. 1 and 2, the furnace 10 is cylindrical and may havean internal diameter of 2 metres and a length of 5 metres, thus a lengthto diameter ratio of 2.5. Respective end walls 12, 14 are provided atthe feed and discharge ends of the furnace 10 and respective burners B2and B1 are mounted with their jets projecting through these end walls.As is clear from FIG. 2, the burner jets are mounted opposite each otherin the upper regions of the side walls 12, 14, so as to face each otheralong the length of the furnace 10. They thus form a co-operatingregenerative burner pair, and the flames and combustion products which,in use, issue alternately from one then the other do not impinge on thematerial being treated which travels along the bottom of the furnace.

The furnace 10 is downwardly inclined from its feed to its dischargeend. The angle of inclination may be in the region of 5°. Thecylindrical walling of the furnace 10 is rotatable relative to the endwalls 12, 14 about its longitudinal axis. In this embodiment rotation iseffected by the action of a driven toothed wheel 16 upon a similarlytoothed thrust collar 17 which encircles the exterior of the furnaceapproximately centrally. For stability, the furnace is mounted upon twofurther pairs of passive rollers 18 towards each end thereof.

At the feed end, two feed channels or chutes 19, 20 are provided throughthe end wall 12, one (19) for the material to be expanded, which, forexample, may be particles of slate and one (20) for an admix, forexample of sand in the size range 200-500 microns, to minimize adhesionof the material under treatment to the furnace interior as previouslyexplained. It is preferable to admit the sand prior to the slate, asshown, to maintain a lining of sand on the furnace walls.

At the discharge end of the furnace 10, an aperture in the end wall 14leads into a sieve separator 22 for separation of sand from the expandedproduct.

In any practical embodiment, the angle of inclination to the horizontaland the speed of rotation of the cylindrical furnace 10 will be adjustedto promote an optimum degree of tumbling of the material so that thereis no violence of motion. In this example, the material under treatmentand the admix travel down the length of the cylinder in an orderlyfashion in about 6 minutes. Furthermore, the rate of feed should beadjusted so that just after the material and admix enter the furnace 10they occupy only about 4% of the cross-sectional area of the furnace.The admix will tend to fill up the voids between the individual piecesof particulate material being treated and will thus not add materiallyto the overall volume. A suitable amount of sand to be added might be20% of the charge by weight. Thus every 1000 kg of particulate materialwill be accompanied by 200 kg of sand.

In use, the burners B1 and B2 operate alternately, as previouslydescribed, and because of the short length chosen for the furnace 10,their combustion gases raise the interior space of the furnace to atemperature of about 1200° C., which is substantially constant along theentire length, excepting for a slight inevitable lowering at the feedand discharge ends. As the material tumbles through the bottom region ofthe furnace, well below the passage of combustion gases between theburners, B1 and B2 it is rapidly heated, principally by an intenseradiant flux from the space and lining above, and also from the hotlining of the furnace 10 passing beneath, to its fusion temperature, andit is thereby maximally expanded.

The use of a regenerative burner pair B1, B2 to achieve a substantiallyconstant temperature along the furnace has the added advantages of fuelefficiency inherent in use of such burners.

After discharge, the sand is separated from the expanded slate productin the sieve separator 22, and to increase efficient use of fuel it canbe recirculated hot, as indicated in FIG. 1. For example, it could bereturned to the furnace at 600° C.

Also, the material fed into the furnace via channel 19 is preferablypre-heated, e.g. to 400° C., to reduce the lowering of the furnacetemperature at the feed end and at the same time enhance the efficientuse of fuel supplied to the furnace. This could readily be accomplished,as indicated in FIG. 1, by channelling hot air from a cooler 24, intowhich the product falls, to a rotary countercurrent heat exchanger 26,where it is used to pre-heat the cold particulate infeed material. Thehot air will initially be at a temperature of about 700°-800° C. and itcan be exhausted from the heat exchanger 26, via a cyclone, through astack (neither of which are shown).

Another particularly effective way of transferring heat from the hotproduct to preheat the infeed material, is again to use air as atransfer medium, but within two rotary cylindrical vessels arranged intandem. The method and apparatus associated with this are best explainedby reference to FIG. 3.

Rotary cylinders 2 and 4 are lined with heat resistant insulatingmaterial and are rotatable about their longitudinal axes which areinclined by a few degrees to the horizontal. Each cylinder is supportedby rollers 6 which can for each cylinder be driven independently i.e.each cylinder can be rotated independently each at selected speed.Thrust collars and rollers are provided to keep the cylinders 2 and 4 inposition axially--these are not shown in the drawing.

Each cylinder acts as a countercurrent heat exchanger. Hot productdirect from the kiln 10 is fed in to cylinder 2 by chute 8 at atemperature of 1000° C. approximately after having been cleansed ofsand. Air is blown along the cylinder 2 by forced draught fan 40. Thecylinder 2 is longer than shown (see break in sketch) and is fitted withbaffle means to tumble the material in the air flow and assist heattransfer. By the time the air reaches the charge end of the cylinder 2it will have been heated to a temperature of about 700° C. and when theproduct reaches the discharge end it will have been cooled. Air massflow rate is chosen carefully to balance solids mass flow rate formaximum extraction of heat. Air velocities are kept relatively low.

The other cylinder 4 acts in a similar way but the heat flow isreversed. Again, it is longer than shown (see break in sketch). At thecool end, which is the feed end, incoming feed material is fed via chute42 and this material progresses down towards the hot end, again incountercurrent, meeting progressively hotter air until discharge. Againbaffle means are provided to assist heat transfer. This cylinder 4 has alarger length to diameter ratio than the cylinder 2 and air velocitieswithin it are thereby increased. Higher air velocities will carry awaydust, but as the particles are unexpanded and of normal density theparticulate material in transit will be able to stand comparatively highair velocities without ill effects--such as being blown backwards. (Incylinder 2 high velocities cannot be tolerated as the expanded productis blown along quite easily).

At the cool end of cylinder 4 the blown air originating from fan 40 isexhausted by fan 44 through cyclone 46.

Sealing arrangements for the cylinders 2 and 4 are important inachieving high thermal efficiency.

At the forced draught end, seal 45 will probably be radial, possiblywith lightly loaded brushes pressing on the cylinder 2 externally. Ifthere is leakage outward it will be of air that is quite cold and heatlosses will not be particularly significant. At the product outlet theproduct itself provides a form of closure or seal by virtue of mound 48which protrudes upwards into discharge duct 50. As a result, most of theforced draught air from fan 40 will be constrained to flow along thecylinder 2.

At the other end, the exhaust, there is a seal 52 of similar form to theseal 45. At this seal the space within will be under a negative pressureunder the influence of exhaust fan 44. Any air leaking in will merelydilute the exhaust air temperature and is of no consequence thermally.

In the centre are two seals 54 one for each cylinder 2, 4 and these areimportant as hot air leaking out, or cold air leaking in will lead toserious heat losses. The seals will probably be radial and of similarform to seals 45 and 52. To keep leakage to an absolute minimum in thishot zone, the pressure differential between internal pressure andatmospheric is kept to a minimum. By adjusting fans 40 and 44 by meansof speed control or by dampers it is possible to adjust the internalpressure in space 56 to atmospheric or very near. Space 56 communicateswith the main furnace and sand stripper and it is important also toavoid any significant air flow within chute 8 in either direction. Underthe influence of one fan alone, space 56 would be under either positiveor negative pressure relative to atmospheric--with attendant undesirableeffects.

Centrally below the space 56 preheated material is collected and storedin a highly insulated vessel 60 with sluice valves 62 at top and bottom.Preheated material at 400°-500° C. can then be transported within thevessel 60 and similar vessels to the feed section chute on the mainkiln. Whilst changing vessels the sluice 62 at the bottom of the lowerconical portion 58 of the space 56 is closed so material can collect inthe portion 58 while a replacement empty vessel 60 is connected. Onceconnected, the sluice 62 is re-opened and the collected material freelyfalls into the vessel 60 to be followed by a stream of material directfrom the cylinder 4.

The above method achieves good thermal efficiency with preheat to400°-500° C. being possible, thereby enhancing fuel efficiency of theprocess as a whole considerably.

Exemplary operating figures are as follows:

The weight of material contained within the furnace 10 at any one timemay be approximately 0.7 tonnes, and with a residence time of 6 minutesthis equates to 7 tonnes per hour. Using the regenerative burners at anassumed efficiency of 78%, heat losses at 15% and with feed and admixadded preheated, as just described, the energy needed will be in theorder of 280,000 kilo-calories per tonne. This represents a substantialimprovement in efficiency over conventional rotary kilns.

The bulk density of exanded aggregate produced from e.g. 10 mm slatechippings or clay/slate pellets will be in the order of 380 kg per cubicmetre.

In FIG. 1, after the cooler 24, the expanded product can pass, at atemperature reduced to about 200° C., to a grading unit 28, which mayinclude a crusher, to provide a size graded product. The cooled product48 in FIG. 3 may be dealt with in a similar manner.

In a second example of the method of the invention there is nopreheating of material before treatment in the kiln. This maximisespossible expansion and produces an ultra lightweight product with evenbetter insulating properties. The weight of material within the furnaceat any one time may be approximately 0.25 tonnes, corresponding to anoccupancy at feed of 11/2% of the sectional areas of the kiln. With aresidence time of 31/2 minutres this equates to 4.3 tonnes per hour. Ifregenerative burners are again used with an assumed efficiency of 78%and with heat losses at 15% and with admix only preheated as previouslydescribed, the energy needed will be in the order of 400,000kilocalories per tonne. The bulk density of the resulting aggregate willbe less than with the first described method for the same size oforiginal feed particles, usually 300 kg per cubmic metre or less. Inthis method (without preheat), after passing through the sieve separator22 a different disposal route can be used, if desired, by passing thecooling/preheat arrangements as shown.

It must be emphasised that the precise details and figures given in theforegoing description and shown in the accompanying diagram are only byway of example, so that the invention can be better understood. Manyvariations are possible without departing from the principle features ofthe invention, namely the provision of a furnace which is of muchshorter length than hitherto and is heated from one or both ends so asto achieve almost an isothermal chamber. By limiting the infeed volumeof material, but having the material pass through rapidly, the degree ofproduct expansion achieved is maximized. Furthermore, considerable fuelefficiencies are possible by using the heat contained in the flue gasesto preheat the combustion air. Many methods of doing this are known.Additional measures such as those described in the example to preheatthe feed can be taken to further improve the efficient use of fuelsupplied.

Specific details which may vary in other embodiments include the meansof rotation of the furnace, which could, for example be achieved by gearteeth around the exterior intermeshing with a drive gear, and the admixmaterial which may be any suitable inert granular material ofappropriate size.

The method of the invention is not solely confined to the heat treatmentof clays, shales and slates, but it can be applied to any rock type thatis capable of being expanded by heat treatment. When the feed is in theform of pellets made from finely ground material, it is possible in themanufacture of such pellets to admix gas producing agents to improveexpansive properties. Such agents are usually carbonaceous and typicallylimestone in finely powdered form can be added in small quantities. Thiscan be useful where the inherent expansive properties of a feed materialare low.

What is claimed is:
 1. A method for heat treating expansible material toform lightweight aggregate in a rotary kiln having a heated interiorspace of a length, breadth and cross-sectional area defined by a kilnlining, and which is inclined downwardly from a charge end to adischarge end, comprising the steps of:a) providing a kiln having amaximum ratio of said length to said breadth of 5:1; b) heating saidinterior space by means of at least one burner directed into saidinterior space from at least one end of said kiln, said interior spacebeing heated thereby to a substantially constant elevated processtemperature, said temperature being substantially constant along saidlength; c) passing expansible material from the charge end to thedischarge end along the length of said heated interior space, saidmaterial occupying a maximum of 10% of the cross-sectional area at thecharge end and being subjected almost immediately to said processtemperature upon entering the interior space, with heat beingtransferred to the material as it moves along said length primarily byradiation from the heated interior space and the kiln lining and bydirect contact with the kiln lining, whereby said material is heated toincipient fusion in passing from the charge end to the discharge end,producing gas which is trapped internally, to form said thereby saidlightweight aggregate.
 2. A method as claimed in claim 1 wherein theinterior space of the kiln is heated by means of burners directed,respectively into the charge end and into the discharge end of the kiln.3. A method as claimed in claim 2 wherein the burners comprise a pair ofregenerative burners, which operate alternately.
 4. A method as claimedin claim 1 wherein the at least one burner is positioned towards a topportion of the kiln.
 5. A method as claimed in claim 1 wherein the kilnis rotated at a speed of rotation and inclined downwardly at an anglechosen so that the material passing therethrough has a transit time ofless than 20 minutes.
 6. A method as claimed in claim 5 wherein thetransit time for material passing through the kiln is less than 10minutes.
 7. A method as claimed in claim 6 wherein the transit time formaterial passing through the kiln is 6 minutes or less.
 8. A method asclaimed in claim 1 where heat is recovered from flue gases external tothe kiln and is used to preheat combustion air.
 9. A method as claimedin claim 1 wherein heat is recovered from lightweight aggregate leavingthe discharge end of the kiln and is used for preheating material to befed into the charge end of the kiln.
 10. A method as claimed in claim 9wherein lightweight aggregate leaving the discharge end of the kiln isfed through a first, downwardly inclined rotary cylinder incountercurrent with cooling air, which air is then passed through asecond, upwardly inclined rotary cylinder in countercurrent with feedmaterial to effect preheating thereof.
 11. A method as claimed in claim10 wherein the first and second cylinders are arranged with a feed endof the first cylinder substantially in alignment with the discharge endof a second cylinder so that a stream of air which cools the lightweightaggregate and subsequently heats the feed material readily flows fromthe first to the second cylinder.
 12. A method as claimed in claim 10wherein the discharge end of the first cylinder is closed off by anaccumulation of cooled product in order to maintain adequate air flowrate through the respective cylinders.
 13. A method as claimed in claim1 wherein sand is fed into the kiln along with the material to betreated.
 14. A method as claimed in claim 13 wherein hot sand isrecovered from the lightweight aggregate leaving the discharge end ofthe kiln and is reused while hot for feeding at the charge end of thekiln.